System of pulse transmission



Dec. 31, 1935. P, FARNSWORTH 2,026,379

SYSTEH OF PULSE TRANSMISSION Filed Dec. 4, 195o 2 sheetsi-sheet 1 AL I /5| PH/LO 7'. FARMSWORTH.

BYwfM ATTORNEY Dec. 31,y 1935. P. T. FARNswoRTH 2,025,379

SYSTEM OF PULSE TRANSMISSION A Filed Dec. 4, 1930 2 sheets-sheet 2 HW/ENTORT PH/LO 7'. FARNSWORTH. BY, I I

ATTORNEY Patented Dec. 3 1, 1935 UNITEDI STATESPATENT OFFICE. ,l

SYs'rEM oF rULsEJrRANsMIssIoN yPhilo T. Farisworth, san/Franciscdfcm., es; .signor to Television Laboratories, AInc., San

Francisco, Calif.,l acorporation of California Application December 4, 1930, Serial No. 500,092 11 claims. (ci. 17a-6) v Tins invention relates primarily to the elec- Figure 5 is a curve representing the sum of protrcal transmission and reproduction of pictures. portional parts of each of the lcurves of. Figure but it is also applicable to other cases where 3, and Figure 6 is a similar curve representing it is desirable to transmit pulses of steep wave the sum of two of .the curve of Figure 3. 5 front through mediawhich tend to suppress the Figures 7 and 8 are diagrams respectively of high frequency components of such pulses, there- .the transmitting and receiving apparatus in a by converting the steep wave fronts into waves radio television system,

of gradually sloping front. Figures 9 and 10 are diagrams of modified Among the objects of my invention are: First, forms of dierentiating networks. 10 to provide a method of reducing the band of fre- Asis well known, electrical picture transmis- 10 quencies which it is necessary to' transmit in orsion, whether it bethe transmission o f still picder to produce an electrically transmitted pictures as in telephoto, facsimile transmission or ture having a given amount of detail; second', to.- of television, the picture is scanned by moving an provide a method of transmitting television pic-,- aperture progressively over the entire area of the tures having satisfactory detail over wire lines picture, in order to generate a current which is v15 which are capable of transmitting but a limited at all times proportional to the reected or transband of frequencies; third, to provide a television .mitted illumination 'of the area of the picturebemethod which will permit the transmission of ing scanned. Since the aperture has anite area television pictures upon radio channels which are Ano details can be transmitted which .are smaller ..20 comparable in width to thosev used for sound than the aperture itself,andthisfactisthe funda- .'broadcasting; fourth, to provide a method of mental limitation of television and other forms picture -transmission which will allow facsimile of electrical picture transmission by all methods telegrams, tele-photography, photo-radio, and now known.

the like, tov be transmitted upon frequency bands In the past, this scanning has usually been acand at speeds which 'are of the same order of complished by the Nipkow scanning' diskfand 25 magnitude as those required for ordinary telemechanical limitations upon the feasible size of graph code transmission, instead of the wide the disk and the speed with which it is required bands or slowv speeds now required; fifth, to proto rotate, which must be `sixteen times per sec'- -vide a method of increasing the speed of transond or more in order to transmit a ,substantially 30 mission of telegraph lines and cables; sixth, puriiickerless picture, have limited the number of 30 suant to the above objects, to provide a method picture elements t0 approximately 2.500; 156i the of converting an electrical wave or pulse having scanning aperture hasbeen about 1/2500 of the a sloping wave front into a pulse which rises to area which it has been required to scan.

its maximum value practically instantaneously; Electrical methods of scanning, such as are Y. and seventh, to provide electrical circuits of sim, disclosed in-my prior Patent No. 1,773,980, re- 35 v ple and practical form for carrying out the methmove this limitation on the number of picture od of my invention. elements transmitted, so that the ratio of aper- Other objects of the invention will be apparture to picture area may be made as small as is l ent or will be specifically pointed out in the -dedesired, and any required amount of detail-may 40' Scription flming a part of this specification, but be transmitted insofar as the scanning operation 40 I do not limit myself to the embodiment of the itself is concerned.` v lnventionherein described, as variousforms may i An analysis of the problem of piivture transbe adopted `within the scope of the claims. mission prwoves that the most usual condition to Referring to the drawings: be met in scanning a picture is a practically in- Figure 1 is al schematic diagram of aY wirestantaneous change in illumination,v i. e., a sud- 45 transmitted television system embodying this inden. transition from dark to lighter vice versa. I vention. f j Considered electrically, this meansthatan elec- Figure 2 isia curve showingthe natureF of the trical pulse must be generated which comprises pulses generated 4by a television transmitter. a'practlcally instantaneous rise (or fall) in curn Figure 3 is a plurality of curves representing a 'rent frombne fixed value of current to another 50 pulse as modified by a filter and derivatives of fixed value. said pulse. Y .f No electrical circuit which it is practical to Figure 4 is a c e illustrating the currents construct is capable of transmitting a pulse of generated by sc an area of gradually this character, which may be described as a 56 changinglllumination. "square `front wave. to any great distance; and 66 -pulses is almost impossible.

and causes them to rise slowly, instead of instantaneously, to their new value.

The Fourier method of analysis shows that the square fronted pulse may be considered as made up of an. infinite number of sine waves having all frequencies up to infinity, and that the slimgishness of response of a circuit is equivalent to its failure to transmit the higher of these frequencies; the lower the frequency of cut-oil?, the more sluggish the circuit, and the more gradual the current rise. This results in great loss of detail in the received picture where the high frequencies are not transmitted. Sharp lines are converted into wide, gradually shaded bands, and small details are passed over by the scanning device before the sluggish circuit can respond.

To transmit a picture approaching an optically reproduced motion picture in detail, requires frequencies of the order of 2,000,000 cycles per second, and at the present state of the art, it is an impossibility to provide a land line which would transmit frequencies of this order, together with low frequencies, to any great distance and without frequency distortion.

Moreover, where it is desired to transmit television pictures by radio, the enormous number of frequencies which must be transmitted necessitate very wide side bands, and, even if one of the side bands is suppressed, very wide communication channels. If a television transmitter were assigned to the broadcast band, the entire spectrum now allotted to that service (545 k. c. to 1,500 k. c.) would be inadequate to transmit a picture having the amount of detail above specified, and it has therefore been necessary to use carriers of the order of 60 megacycles in order to obtain channels of the requisite-width.

on on these ultra high frequency channels is, as yet, unsatisfactory.

Closely analogous in eifect to the sluggishness of the circuits is the distortion due to the finite size of the aperture. In its steady passage over the picture ileld, its response to a change in illumination begins at the instant its front edgel reaches the change. but the response is not complete until its rear edge also reaches the change, and the entire aperture covers the region of changed illumination. This makes the v wave front more sloping and attenuates the high frequency components. The effect is known as "aperture distortion" or aperture effect.

The sameterm is applied to a similar eiiect occurring at the receiver. The'incoming picture is traced by an area of illumination which moves across the picture ileld in the same mannerv as the transmitting aperture, and the apparent illumination of any point on the field is the mean illumination of that) area as it passes the point.

- For this reason, if the illumination of the area v be suddenly increased, those points near the trailing edgeof the area which receives the increased illumination for only an infinitesimal time, appear less bright than those which are near the leading edge when the change occurs,

and which receive the increase during the entire' time the area is traversing them.l Again the ei'- fect is to spread sharp lines into shaded bands and spoil detail, and although this has no effect. on the transmitted frequencies, its effect uponV the eye is the same as would be produced by a further suppression of the high frequencies.

It is to be noted that the frequencies here mentioned are the component frequencies of the pulses, and that the pulses themselves come, in general, at random. The only truly periodic components in the transmission of a picture are the picture frequency itself (say 16 cycles per second) and the scanning frequency, or, as they are sometimes referred to, the lowerv and upper scanning frequencies. With this in mind, it will be'realized that what must be transmitted, in yorder to produce good pictures, are these two truly periodic frequencies, on which are superposed random pulses or transients, and that the difculties of transmission relate almost wholly to the latter.

In transmitting `or amplifying the transients, as has been pointed out, the steep wave fronts or instantaneous current rises, are converted into sloping wave fronts or gradual current rises. None the less, considered from the receiving end, there is a definite instant of arrival of the wave, which is the instant when the change in current starts to take place. At this instant the rate of change of current, (mathematically its first derivative with respect to time, or di/dt) changes from zero to some finite value. The rate at which I the.current is changing has a discontinuity or instantaneous change of value at 'this point.

,'If there is phase distortion or envelope delay in the circuit, even the rate of change may be gradual, and there will1 inthis case be no `Adiscontinuity in the first derivative di/dt. 'I'here is still an instant ofv rst arrival of the wave, however, and this will bemarked by a discontinuity in the second derivative or in some of the higher derivatives. Where a derivative exhibits a discontinuity, all of the higher derivatives have infinite discontinuities, changing from zero to plus or minus infinity at this instant. If the pulses follow each other so clo'sely that the first change is not complete before the second arrives, the change of .slope is no longer from zero td a finite value but from one nite value to-another or to zero, but the discontinuity at l the instant of arrival remains.

While the wave has thus far been considered as a current wave, it will be realized that the above holds equally true for a voltage wave,- and this should also be kept in mind in what follows. The transformation theorem which states that in any equation dealing with voltage and current these quantities may be interchanged, and by, substituting admittances for impedances the Aequation still holds, is true here as elsewhere. 'I'he term current has and will be used in this specification, in order that a clear and connected description may' be given without repetitions explanations, but the transformation theorem mentioned, and other similar well known transformations, indicate the breadth of equivalents involved.

Considered from one aspect, the broad invention here presented is a method of communicationby electrical pulses which includes the steps of modifying the pulses by removing certain fre-l quency'components therefrom, transmitting the modified pulses, and adding, at the receiver, components substantially equivalent to those -previously removed to restore their original wave form.

From another point of view, the broad invention is a method of converting waves or pulses having sloping wave fronts into steep fronted waves by adding thereto components proportional to one o'r more derivatives of the arriving pulse with respect to time, at least one of the added derivative components preferably having a discontinuity at the instant of arrival of the pulse.

The derivative components are supplied by a differentiating network. which, in its preferred form, comprises a relatively large impedance for determining the current ilow and establishing the relation oi. current and voltage waves in the network, and a shunt arm of relatively negligible impedance, wherein the voltage drop leads in phase that in the control impedance, and across which is developed a voltage proportional to the derivative of the arriving pulse'. If a vacuum tube be connected with its control circuit across this shunt arm, its output current is proportional to the voltage in this arm, and this current may be circulated through a similar network to generate a higher derivative. By adding proper components of current or voltage proportional to the arriving wave and its derivatives, including a derivative having a discontinuity at the instant of arrival of the wave, square fronted pulses are produced.

As used in television lor other electrical picture transmission, these reconstructed pulses are used to control the illumination ofthe area whichl scans the field of the received picture, giving detail whichis even better than that produced by a receiver connected directly to the trails-l mitter, since the aperture distortion of the transmitter is completely corrected. .By adding a still higher derivative component theA aperture distortion at the receiver is largely compensated, since the powerful pulse corresponding to the infinite discontinuity at the instant of arrival gives a sharp outlineabout the pictured masses, pro-l ducing an illusion of relief.

There will be next described in detail the application of my 4 mitting system, following which a .mathematical explanation of its operation will be given, and its application to other uses .will be pointed out.

'I'he television system shown in' Figure l is a simplified-diagram of a device utilizing the transmitting and receiving apparatus disclosed in my previous Patent No. 1,773,980, .and in mycopending applications,

and 461,111.

The essential feature of this transmitter is a special type of photo cell comprising an en-v velope 2,.. preferably cylindrical in form, and

having in one end a plane window 2. Throughv window a lens system' 3 focuses an image this of the object 5, upon a4 photo-sensitive cathode whose picture is tdbe transmitted, plate t.. Parallel to and closely adjacent to the cathode is a screen anode 1, which is maintained. at a vpotential posil tive to the cathode by means of a battery or other source 8.

ode are vfocused ata corresponding point in the inventionto a television trans- Serial NOS. 270,673, 449,984,

liberated from the cathode in plate of the tube plane of an aperture I I ina small tubuluar shield I2l which projects inward from the side of the envelope adjacent the window. Within the shield is a target or collector I3 which is preferably maintained slightly positive with respect to the shield by means of the source 8, and which collects those electrons which fall upon the aperture. Since electrons are liberated simultanously from the entire area of the image. there is thus formed a complete electron image of the object to be pictured, and if a fluorescent screen were placed in the tube in the plane of the aperture, the picture would appear upon it.

The electron stream which forms the electron" image is deflected magnetically by means of coils I6, and serves to deflect the' image across the' aperture at the low scanning frequency. A

' similar oscillator operating at the high scanning frequency supplies a set of coils arranged at 90 to that shown. This is omitted from the dia-A gram for the sake of simplicity. I The two deections of the image cause it to move 'past the aperture so that the latter traverses the entire area of the picture progressively to accomplish the scanning.

In the preferred method, saw-tooth oscillations are'used for the vertical movement of theimage; which is moved uniformly downward over-the aperture in about 115th second, and then returned to its original position practically instantaneously. At the same time, the other oscillator deilects the image from side to side at substantially constant speed, equivalent to from 2,000 to 4,000 cycles per second. so that electrons enter the aperture consecutively from each unit area of the picture field as projected on the cathode, and

efall upon the target. l

The electron current collected by the target I3 passes through a resistor I1, causing a potential drop across this resistor which actuat'es an amplifier I8.

The part of the apparatus thus fiar described may be identical in construction and operation with that' disclosed in my U. S. Patent No. 1,773,980- and applications above mentioned. Y The amplifier I8 is shown as feeding into a low pass lter which removes all of the frequenii above a predetermined cut-olf value. The lllter 'is shown as feeding into a line 2l. The reasons for using the filter will be discussed `in detail below.

'Connected' in series with the line isI a resistor 22, andthe -line is closed through an impedance comprising an -inductor 23 and a variable resistor 25 .in series. If the ohmic resistance of the line .is sufficient so that the current in the line is determined bythe line resistance rather than the closing impedance, the resistor 22 may be omitted.

vThe important factor in the network as here dis-` closed, is that the impedance fromwhich the network comprising the elements 23 and 25 is fedbe primarily' resistive and of a sui'ilciently high value so 'that the effect of the impedance of elements 23 and 25 on the current ow is negligible.

Connected across vacuum tube 28, preferablyv of the screen grid type, so thatits output impedance is high. The 26 is connected through another network comprising an -inductor 21 and a variable resistor 28. t

' In a laboratorycircuit as actually constructed the-closing impedance is a' I for s wave which' 'had through' 'a maar designed for substantially complete cut-o2 above 7 k. c.. the resistor 22 had a value of 1/4 megohm,

and the inductors 23 and 21 a value of about 3ll millihenries each. Setting the variable resistors 25 and 23 at about 2,000 ohms gave the best ap` pearance to the picture. The impedance of the line was negligible as compared to that of the control resistor 22.

Coupled to the plate circuit of the tube 2l through a condenser is a cathode-ray receiving tube or oscillight 3|, whose grid 32 connects to the condenser 30. The oscillight has a hot filament 33 which releases electrons to be accelerated by the anode 35 and focused in a spot upon a fluorescent screen 30 in the large end of the oscillight. The intensity of this spot is controlled by the grid 32, and it is deflected, preferably magneticallmby the coil 3l supplied by an oscillator 3l, in a manner to' correspond with the deflection of the electron stream by the transmitting scanning oscillator. This causes the transmitted picture to appear upon the iluorescent screen, traced by the focal spot. 'In this case also, but one of the scanning oscillators is shown. -'Ihe receiving scanning oscillators are held in synchronism by being connected to the oscillight grid 32 througia resistor 40, as is described in my above mentioned application, Serial No. 461,111, the scanning frequency components in the transmitted wave being suillcient to hold the oscillator in step.

If the amplifier I8 be connected directly to the grid 32, and if it capable of amplifying a broad band of frequencies,` say ofthe order of 300 to 1,000 kilocycles, an excellent reproduction of the object 6 will appear upon the fluorescent screen 30 of the oscillight. The introduction of the filter 20 into the line, without the addition of the other apparatus between the filter and the oscillight, converts the clear-cut picture upon the screen into a blurred image which is almost unrecognizable. The sharply `defined edges are spread out into wide, shaded bands', and the picture assumes the general appearance of a dow cast upon a distant screen by a. large light so ce; i. e., the dark portions of the image are surrounded by a large penumbra..

The insertion of the network' already described between the filter-and the oscillight at once restores the detail of the picture, the denition being even better than that obtained by connecting the amplifier directly to the grid of the oscilliaht. Practically, enormous advantages are obtained by filtering o u't the high frequencies before transmission. The filter need not be a'separate structure as shown, but may be incorporated in the amplifier, giving 'a cheaper, smaller, and more eilicient structure than that necessary for handling the wide bands heretofore required. Moreover, if the transmitted band be restricted in width, the line` may be utilized for carrier telephone (or television) purposes, with consequent savings in investment. In addition to these factors, the eect of phasedistortion or envelope delay is minimized. since thiseifectis a function of the frequencies transmitted. Where these frequencies are but one or two percent of those otherwise required, the undesirable effects pro'- duced are reduced in approximately the same proportion. and their correction is rendered correspondingly easier.

'I'he filtering out of the high frequencies is even 75T more necessary where transmission is overa radio filter, as was above stated.

channel, as indicated in Figures i and 8. since the wide band formerly necessary is difllcult to modulate upon any but the highest frequency carriers,

-diillcult to receive without side band cutting, and

cause excessive interference with other services. 'In the radio transmission system illustrated in Figure 7, 'the apparatus utilized'is identical with that used-for line transmission up to, and including the filter 2|I',ani is indicated by similar reference characters, distinguished by accents. Instead of feeding into a line however, the filter feeds into an oscillator-modulator 4I of the same type as is required for sound-radio transmission, and is radiated from an antenna 42.

-At the receiver shown in Figure 8, the signals are picked-up by the antenna 42', and passed through the amplifier-detector 43, and after detection are impressed upon the grid of the highy impedance tube 44. The plate circuit of this tube comprises an inductor 23' and resistor 25', which have the same function as the similarly designated elements in Figure 1, the resistor 22 being unnecessary in this case since its function is filled by the plate impedance of the tube. From this point forward, the circuit is identical with that shown in Figure 1, with the exception of the gridbiasing resister 45 and grid condenser 45', which are used in accordance with usual amplifier practice, and which are unnecessary in the line terminal apparatus where the tube 44 is omitted.

The action of the terminal network in restoring the picture can best be understood from a mathematical analysis of the picture pulses and the distortions which they undergo between the transmitter and receiver. characteristically, the changes in illumination in a picture are sudden; i. e., the aperture, traversing a' portion of the picture having a constant illumination 1c, suddenly arrives at another portion having a' different illumination h. The instant at which this change in illumination occurs may be taken as time t=0. The result is a current in thc resistor Il which has, up to time 0, a value proportional to k, and after time 0 a suddenly increased value proportional to h. The form of this current pulse is illustrated in Figure 2` If the scanning aperture be large, the increase in current from 1c to h, ins ead of being instantaneous, occupies a defini time T. the increase in current occurring at a constant rate' during this time. This gradual increase is shown by the dotted curve 46' of Figure 2, the condition for an infinitely small aperture being shown by the solid curve 46. With the electrical scanning system however, the aperture may be made as small as desired, approaching very closely the conditions indicated by the curve 48. For the sake of simplicity in the analysis, only 'the condition introduced by the infinitely small aperture will be considered, since the eectof a larger aperture is substantially the same las that of a low pass If we consider the current 1c as a direct current, on which are superposed the pulses comprising the picture current, we may neglect k and represent the pulse by a Fourier integral where A is an auxiliary variable of integration. .The value of this integral from t= to t=0 is 0, and from t=0 to t=o it is (h-Ic). For posaosas're itive values of tit may be expressed in the. simplied form:

(D o 1=`f(f)=f do ein of I (1i-k) ein ma.

The integration with respect to u indicates the summation 'of all the component frequencies of the pulse, while the integral with respect to X gives the amplitude of these frequencies. Per-A forming the first integration and simplifying,

1=f()=2( h" k) f "dm 'I' o w This represents the original pulse. If we remove `from it all frequencies above the modified pulse can be expressed by the same integral by changing the upper limit from w to N. giving This function is plotted against Nt as curve 41, Figure 3. It will be noted that the current I is zero up toltime t=0, 'when it suddenly starts to rise, having its steepest slope at this point. It continues to rise with gradually decreasing slope, to a value greater than its nal value h-k, and then falls to its iinal value" through a series of damped oscillations about this value. This is vthe form of the incoming wave at the receiver.

In interpreting this curve, it should be realized that N is merely the value of o atthe cut-oi! frequency and that the curve holds true whatever the value of cut-oli. Dividing all of the abscissas by Nwouldy convert the X axis into a true time axis for the one particular value of cut-oil chosen.

In the complete analysis, the function. being odd,` passes thru values prior to t= which are equal and opposite to the values assumed for corresponding positive times, i. e., f(t) 1=f(t) This condition cannot obtain'in any physical network, however, since it would necessitate that vthe filter anticipate each pulse, going'into oscillation in. response to the pulse an iniinite time before its arrival, or elsethat the pulse `be in.

iinitely delayed inpassing thru the network. What actually occurs, in a system without appreciable phase distortion, is shown by curve 41, which corresponds with oscillograph traces obtained under the'conditions given. 'A deiinite 'discontinuity in slope, at the instart of arrival voi! the pulse, is ycharacteristic of any physical network which can be constructed.

- This abrupt change of slope represents a sud-- den rise or discontinuity in the derivative I' of the curve, which is plotted as curve Il of Fig-- ure3. Aswillbeseemthisfunctionisalsosero up to time t.- 0, when it rises abruptly to a magimum value, and then falls gradually to a value I'=0 through A a series voi.' oscillations approximately 90' behind the oscillations of I.

The second derlvative I of curve Il is plotted `as curve 49 ofthe same iigure. It rises tota -va1ue`1'f= at time o, railelnstantly' to aero value,` and dies out through a series of damped oscillations lagging about reo' behind those or I.

of curve'l'l and a proportional amount of curve 48 such that the initial value oi' I' at 4t=0 is equal lto the final value of I, the new curve will be an` abruptly rising pulse, substantially equal to the original pulse plus a small damped oscillation, as indicated by curve 50, Figure 6. Add-l ing to this curve a smaller proportion of curve 49 slightly decreases the amplitude 'oi' the superposed oscillation, and adds an instantaneous high peak to the steep front of the wave, as indicated by curve ."I` of Figure 5.

The eireet of the terminal network described above is to add components proportional to the first and second derivatives to the incoming current wave. so that curve 5I represents the resulting wave as applied to the grid I2 of the cathodeg ray receiver."

'I'he current I, iiowing through the resistor 25,

causes a voltage drop IR, where R is the resistance of the element 2l. The sainev current through the inductor 2l, produces a voltage drop equal to y where L is the inductanceof the element 2o. The

sum of these two voltages is impressed upon the grid'of the tube 26.

'I'he quantity y I r df.

however, is thesame `as that whichv we haveal-` ponent I'. 'Ihe same currentrin the inductor 21 produces a voltage proportional to I' plus a component I". The I' components are additive, and hence the voltage applied to the grid of the oscillight'corresponds to the'curve 5I already described. l. .f v-

'I'he two resistors 25 and- 28 being variable, the relative values `olf the various components may be easily adjusted until the picture assumes its best appearance.

Up to this point, we have considered vonly abrupt changes inillumination and current.

Such changes vcomprise avery large majorityv of the4 impulses occurring in plomo-transmission,- but ina few cases there isa gradual change of illumination of light to dark -or the reverse.

vThese changes also'. usually ,involve an abrupt change of slope at their beginning or end, whichcause steep fronted lderivativewayes which are, however, of less magnitude than those produced by a lpulse of the fundamental type. Without considering the mathematicsv of such` a condition in detail. the general effectls shown in Figurefd.'

'In this figure, curve 52 indicates a currentwhicn gradually "Y10,

issteadyuptotimet1.rises t1 and tz, and then resumes a steadystate ata new value. The form in which thiscm're'nt would be restored by the network'here shown, is indi- -cated'l'ouhly by curve ll.- The change in causes a steep fronted pulse, which raisesA the level of the lentire current at t1. Thefundamenpicture current level to follow the level of the tially undistorted. The effect is curve 53. In this curve, thelevel is as much below the true level as it was above in the opposite sweep, and the average effect is substanmuch exaggerated in the figure.

The compensating network differs from the ordinary amplifier, in that the impedance whose voltage drop in applied to the input circuit of the tube does not control its own current flow,

. which is determined by the much greater external resistance 22. The circuit being primarily resistive, the current and voltage waves have the same form, and the voltage across the relatively negligible iductance 23 is therefore proportional to the derivative of both current and voltage waves.

Were the control impedance inductive, the current wave would differ from the voltage wave, being proportional to its integral. The voltage across the inductance would still be proportional to the derivative of the current wave, but it would be identical in form with the voltage wave. This is the condition in ordinary impedance coupled amplifiers. The restoring circuit as a whole, even including the tube 26, usually attenuates rather than ampliiies the received signal, and an additional tube or tubes must be provided where amplication of the received signals is necessary.

The additional amplifier, where required,-may best be inserted ahead of .the network, as in the radio adaptation of the invention, and it may be of any approved type which'is adequate to amplify the received signal band. Its nal tube, however, is preferably, like the tube 44, of the screen grid type, having a plate resistance which may be considered as infinite, this resistance controlling the current ow and acting as the element 22.

The value of the second derivative component in the 'wave lies largely in compensating for aperture effect in the receiver. The sudden large change, coming at the rst change of slope of the pulse, forms a sharply defined edge onvthe image instead of the shaded band the width of the aperture.v As has been stated above, this effect may, if desired, be omitted with only slight loss of detail in the picture.

There are certain cases, however, in which, because of phase distortion or through repeated changes of energy form, such as modulation and demodulation by -sluggish systems, where the 'change of slope t the instant of arrival of the transient' ceases o he abrupt. Even in these cases there is an abrupt r'ate of change of slope; i. e., a Adiscontinuity in the second derivative, which enables the original pulse to be substantially reconstructed. In such cases the third derivative maybe added to compensate for the aperture effect. Thus far, no cases have been found where the need has arisen to add derivatives higher than the third in picture transmission, but there is no reason why this could not be done if desirable. i e

In 'the theoretical consideration of the effect .l of ltering high frequencies from the picture pulses, it has been assumed that the cut off of the filter is' absolutely sharp; i. e., that at the frequency fe there is no attenuation, 'while at fc-(l-Af, the attenuation is infinite. In practice this con- I dition never obtains, the cut-olf being gradual,

upon the crest of the reconstructed wave. It is therefore advantageous that the lter used should i pass uniformly the frequencies up to fc, attenuate the frequencies from fc to some other value f at an increasing rate, all frequencies above f' being substantially eliminated. The conditions of Athe particular problem will determine the width of the band of increasing attenuation between je and f'. the compromise being between band width and fringes on the transmitted images due to the oscillations. Even with the sharpest cut-offs obtainable, however, these fringes have not proved in practice to be a serious detriment. It is because of uncertainties as to the sharpness of cutoif in different transmitting systems, or even in the same system at different times, that it is advisable to provide adjustability in the restoring network, so that the proportion of arriving pulse and derivatives may be changed to give the picture its best appearance. the resistive element in the network be the adjustable one, vor even that Separate elements be used, since a resistor and a variometer in series, or a variometer wound with resistance Wire, would give the effect required.

Other forms of differentiating circuits are also possible. Thus, in Figure 9, there is shown a circuit which will differentiate a voltage wave, although its* general applicability is not as great as that of the circuit already described. In this case the high control impedance is provided by a small condenser B5, the shunt arm being formed by a relatively negligible resistance 66 in series with a large capacity 61. The latter is bridged bya high resistance leak 68 through which the grid of the tube 10 is biased by a battery 1|.

In this circuit the voltage drop across the resistor provides the derivative component, while that across the condenser represents the original wave. Y

Figure 10 illustrates a circuit giving components of both first and second derivatives, as well as the fundamental. Herethe control impedance is again supplied by a condeiser 15, while the condenser 16 gives a fundamental voltage drop, the

' resistor Il a first derivative, and the inductor 18 a second derivative'com nent.

Conditions are rare n which the series resistance in aclrcuit can economically be made sufficiently small to justify the use of a capacitive 4impedance as in these two latter circuits, but they are included to illustrate the possible range of equivalents. In general it may be stated that with any control impedance, a voltage drop leading that in the impedance will be proportional to a derivative, the first if the angle of lead be the second if the angle be Actual experiment has shown that the principal effect of eliminating the high frequencies and reestablishing them as here described, is a slight heightening of the contrast in the picture, bringing out the angles and planes in sharper relief, andgiving a result which several observers have described as a stereoscopic effect. Obviously, no true stereoscopy is produced, and this stereoscopic effect is an optical illusion resulting from the slight distortion.

It is to be noted that with this system of transmission the actual'length of theinterval which separates successive pulses is not important, and

It is not necessary that cut-oif lis the least favorable vfor compensatingthe highest frequency transmitted. Thus, -a pulse of decreasing current may follow one of increasing current long before the final value of the increase has been reached. This results merelyin a sudden rate of change of increase, which is transformed by the corrective network into the semblance of the negative pulse. This was well illustrated in one experiment tried, in whichthe scanning frequency used was 4,000 cycles, the picture current being passed through a lter having a cut-off value of 6,000 cycles. Pictures were transmitted through this system having as many as 20 discrete changes in illumination per cycle. In this case, the cut-off frequency was so close to the scanning frequency that no recognizable picture whatsoever .was produced without the rewas better defined than whenv transmitted directly over a lirie of negligible length.

As was indicated above, the condition of sharp and reestablishing the sharp fronted pulse. If the attenuation is proportional to frequency above some specied value, the first derivativealone is complete compensation of the original pulse. Second derivatives may be added in this case, as before, to compensate for'the distortion of the receiving aperture. It should be mentioned here, that the second derivative also helps to compensate for any distortion caused between the terminal network and the receiving oscillight itself by the capacity of the leads and apparatus.

This capacity integrates the pulses fed it, converting a portion of the second derivative back to first derivative, and rst derivative to fundamental.

While I have described my invention primarily as applied to television signals, for which service I have succeeded innarrow'ing the required band of frequency by a factor as high as one hundred, it is obvious that it is also applicable to the transmission of still pictures, in which it will give comparable narrowing of the band, and also to tele-v graph and cable signals.

I am aware that Carson and others have used differentiating circuits to correct wave form in submarine cables and' thus increase signaling speeds. In Athis case, however, the high frequencies, although attenuated, are not removed, and the excessive phase delay so distorts the waves that many successive derivatives .are necessary to f build up` a steep wave front. It has escaped frequenciesis,

' the contrary, their presence notice in the past that the presence of these high unnecessary and undesirable; on has been deemed I essential to give a true reconstruction of the wave.

Moreovenalthough the use of diiferentiatins circuits for cable use has been known for over twelve years to experimenters in television and picturetransmission in' general, it is by no means obvious that they are applicable to these latter uses, and no attempt has heretofore been made to apply them in this manner. This is'quite probable because of the misinterpretation of the Fourier integralmentioned above.

j It will be seen that this invention, :by reducing the requirements which have heretofore been imposed upon land lines and radio links required for television, removes the outstanding difllcul- 'ties in the way'of a commercially practical'television system. Actual experiments show that satisfactory-.pictures may by this system be transmitted over ordinary telephone lines. Parallel storing network. Witlfrthe network, the picture vspect to time.

vision pictures upon wave bands no wider those now used for sound radio transmission.

Additional detail can be secured by widening the band allotted to the picture, but the detail of apicture transmitted upon any band may be increased in the order of 100 to l. This makes posv sible the transmission byr television of pictures comparable in detail to those projected upon the moving picture screen, and by removing the limitation heretofore imposed by the transmitting medium, throws the burden of development back once more upon transmitting and receiving equipment.

I claim:

1. The method of picture transmission which comprises the steps ,of scanning a picture field to produce picture current corresponding to a predetermineddegree of detail, filtering said current to remove the high frequency components thereof, transmitting said currents, .initiating at the receiving end components substantially equiva` lent to the components removed in ltering, and

combining said components with components I equivalent to the received current.

2. 'I'he method of picture transmission which comprises the steps of scanning a picture eld to produce picture currents corresponding-to a predetermined degree of detail, `transmitting pulses y f2 corresponding to the low frequency components only of said current, receiving ,said pulses, initiating at the receiving end-components substantially equivalent to, the high frequency components of the original picture current, and combining .the transmitted pulses with the locally initiated components.

3. The method of picture transmission which comprises the steps of scanning a picture field to produce picture current corresponding to a predetermined degree of detail, transmitting pulses corresponding to the low frequency components adding to the received pulses components proportional .to a derivative of said pulses with rev4.`The method of picture transmission which comprises the 'steps of scanning a'picture field to produce picture current corresponding to a predetermined degree of detail, transmitting pulses correspondingK to the low frequency components only of said currents, receiving said. pulses, and adding to the received pulses components proportional to a plurality of derivatives of said pulses with respect to time.

5. The method of picture transmission which comprises the steps of scanning a picture field to produce picture current corresponding to a predetermined degree of detail, transmitting pulses corresponding to the low frequency components only of said currents, and utilizing changes of slope in the wave front of the received pulses to create `steep fronted pulses for forming the rec'eivedl picture.

6. In a picture transmission system, means for scanning an object to produce a picture current,

-means for attenuating thehigh frequency com-- pornents` of said current, a communication channel for conveying 'impulses corresponding to the `modied current to a receiving station, means actuated by the received impulses for regenerat-v v received picture.

all

-only of said currents, receiving said pulses, and

ing components substantially equivalent'to the 7. In a picture transmission system, means for scanning an object to produce a picture current, means for attenuating the high frequency components of said current, a communication channel for conveying impulses corresponding to the modified current to a receiving station, means actuated by the received impulses for generating an impulse substantially proportional to a derivative of the received puise with respect to time, and means responsive to said derivative impulse for forming the received picture.

8. In a picture transmission system, means for scanning an object to produce a picture current, means for attenuating the high frequency components of said current, a communication channel for conveying impulses corresponding to the modified current to a receiving station, said channel terminating in a circuit of relatively high'impedance, an element in said circuit wherein the voltage drop leads the drop in said circuit and havingrelatively low impedance with respect thereto, and means responsive to the voltage drop in said element for forming the received picture.

9. In a picture transmission system, means for scanning an object to produce a picture current, means for attenuating the high frequency components of said current, a communication cham.

nel for conveying impulses corresponding to the modified currentJ to a receiving station, said channel terminating in a circuit of relatively high resistance, an inductor in series with said circuit having a relatively low impedance with respect thereto at the highest transmitted frequency,

and means responsive to the voltage drop in said Inductor for forming the received picture.

10. In a picture transmission system, means for scanning an object to produce a picture current, means for attenuating the high frequency components of said current, a communication channel forconveying impulses corresponding to the modied current to a receiving station, said channel terminating in a circuit of relatively high impedance, a closing impedance comprising elements wherein the voltage drop has com- "ponents leading and in phase withv the drop in said circuit impedance and of low impedance relative to said circuit at the highest frequency A transmitted. and means responsive to the voltage drop in said elements for forming the received picture.

11. In a picture transmission system, means for scanning an object to produce a picture current, means for attenuatin'g the high frequency 20 components of said current, a communication channel for conveying impulses corresponding .to the modied current to a receiving station, said Vchannel terminating in a circuit oi relatively high resistance, a closing impedance comprising an inductor and a resistor having substantially equal impedances at the highest frequency transmitted, said impedances being low relative to the impedance of the circuit, and means responsive to the voltage drop in said impedance for forming the received picture.

PHILO T. FARNSWORTH. 

